That representation of a Sudoku is elegant, but I think it is not the most natural representation. The base constraint programming style will use a variable per square with domain 1-9, and then 27 all_different constraints. This representation is a lot closer to how people talk about the rules of Sudoku, which in my mind makes it more natural.

A full MiniZinc program would look like this

    int: n = 3;
    int: s = n*n;
    set of int: S = 1..s;
    
    array[S, S] of opt S: puzzle;
    
    array[S, S] of var S: board;
    
    % Sudoku constraints
    constraint forall(row in S) ( all_different(board[row, ..]) );
    constraint forall(col in S) ( all_different(board[.., col]) );
    constraint forall(r, c in {1, 4, 7}) (
        all_different(board[r..<r+n, c..<c+n])
    );
    
    % Set up puzzle
    constraint forall (r, c in S where occurs(puzzle[r, c])) (
        board[r, c] = puzzle[r, c]
    );
    
    solve satisfy;

And an instance file looks like this

    puzzle = [|
         9, <>,  8,   1, <>, <>,  <>, <>,  4 |
         1,  2, <>,  <>,  8,  6,  <>,  5, <> |
        <>, <>,  7,  <>, <>, <>,  <>,  1, <> |
    
        <>,  8,  3,  <>, <>, <>,  <>,  6,  9 |
         7, <>,  6,   8, <>,  3,  <>, <>, <> |
        <>, <>, <>,   4,  6, <>,  <>,  8, <> |
    
        <>, <>, <>,  <>, <>,  1,  <>, <>, <> |
        <>, <>, <>,  <>, <>,  4,   5, <>,  1 |
         5,  4,  1,   9,  3,  8,  <>, <>, <>
    |];